Ugh!!! Nobody wants to be smelling like a mountain ram goat or a wash cloth that has been left out for a month. I always heard about the dreaded mildew coming after you in your sew-in weaves and whatnot. Even when I got my first weave ever three years ago my stylist warned me about this. Luckily although it was a full sew-in weave and I went swimming and exercising in it quite frequently, I never got any type of ungodly smell from it - mostly because I was sticking my head under a Portable Hooded Dryer every time there was dampness because I was so afraid of mildew.
Nonetheless when I started doing crochet braids I assumed mildew would less likely happen because my hair was not totally hiding underneath a mesh cap and some thick straight Indian hair. Instead with crochet braids it is super light and airy and your scalp is exposed and has room to breathe.
However, it was only when I wore the Long Deep Twist crochet braids and took them out, that was the first time I ever smelled mildew (didn't help my stylist confirmed it for me the next day too). What happened made sense. I went exercising in this long, less breathable hair and even swam in it one time, and didn't dry it properly afterwards (there was no sitting under the hooded dryer like I did when I had the weave). I do not think this would of happened if the hair was shorter since there would be more air circulating through but the fact it was long and less curly kept my cornrows and scalp damp and moist.
To get the mildew out is pretty simple, granted I did this after I took out the crochet braids. I think you will have more of a challenge if you still have the crochet braids in. I washed it in a cleansing shampoo like As I Am Curl Clarity Shampoo to totally strip my hair and I even combined some apple cider vinegar with some shampoo to really massage out the funk in my scalp. Afterwards if you have type 4 hair it will be very, very dry so you must follow up with a good deep conditioner. Afterwards my hair felt and smelled good to go.